The correct answer is Option C: Special structure formed by extension of plasma membrane.

Here's a detailed explanation:

Mesosomes are inward folds or invaginations of the plasma membrane in prokaryotic cells (bacteria and archaea). They are not present in all prokaryotes and are considered artifacts of chemical fixation used in electron microscopy. While they were once thought to be important for various cellular functions, they are now believed to be a result of the preparation process rather than a true cellular structure.

Let's look at why the other options are incorrect:

Option A: Membrane bound vesicular structure: While mesosomes are membrane-bound, they are not vesicular structures. Vesicles are small, membrane-enclosed sacs that transport materials within the cell. Mesosomes are more like extensions or folds of the plasma membrane.

Option B: Chain of many ribosomes attached to a single mRNA: This describes a polysome or polyribosome, which is a structure involved in protein synthesis.

Option D: Medium sized chromosome: Prokaryotes have a single, circular chromosome located in a region called the nucleoid. It's not a "medium-sized" chromosome in the same way eukaryotic cells have multiple, linear chromosomes.

In summary, mesosomes are an artifact of chemical fixation and are not considered true cellular structures. They are formed by extensions of the plasma membrane in prokaryotic cells.